1. Field of the Invention
The present invention relates to a technique for performing interlaced to progressive scan conversion to convert interlaced 60 fields per second video, which has been obtained by carrying out 3-2 pull down conversion on progressive 24 frames per second video, to progressive video.
2. Description of the Related Art
Video signals in various data formats have been adopted to transmit and decode video. Examples of those video signals include an interlaced video signal in a 60 fields per second (including 59.94 fields per second) format and a progressive video signal in a 60 frames per second (including 59.94 frames per second) format. To cut down the quantity of video data to be transmitted and written, moving picture data is often compressed extensively compliant with an MPEG standard, for example.
Meanwhile, as the definitions of display monitors have been increasing year after year, scan conversion is carried out more and more often. For example, a method for converting original interlaced video to progressive video while a video signal is being decoded is known.
Hereinafter, processing of converting a movie film in a 24 frames per second (including 23.976 frames per second) scan format as a material of moving picture data will be described as an example. A broadcaster performs progressive to interlaced scan conversion to convert progressive 24 frames per second video to interlaced 60 fields per second video by 3-2 pull down conversion using a telecine converter. The interlaced 60 fields per second video thus obtained is then input to a moving picture encoder.
The “3-2 pull down conversion” for use in this scan conversion is a technique for reading the first one of two consecutive frames of a film source as two fields of a video signal and then reading the next frame as three fields of the video signal. And such a technique is currently used extensively in the art. Two of the three fields read have the same data. That is why the interlaced 60 fields per second video that has been subjected to the scan conversion includes the same fields redundantly.
A video stream that has been encoded by a moving picture encoder is broadcast and then received by a receiver, which decodes that video stream.
The receiver performs interlaced to progressive scan conversion to convert the interlaced 60 fields per second video to progressive 60 frames per second video. For example, Japanese Patent Application Laid-Open Publications Nos. 2002-330311 and 3-250881 disclose such scan conversion methods. Specifically, the receiver detects the regularity of the 3-2 pull down converted video, thereby locating an image signal interval that has been formed by the 3-2 pull down processing. As used herein, the “regularity” is that at least one out of every five fields of the 3-2 pull down converted video is the repetition of the field before the previous one. In the image signal interval thus located, the scan converter combines together the odd- and even-numbered fields of the same video frame yet to be subjected to the 3-2 pull down conversion. As a result, the interlaced 60 fields per second video can get converted into progressive 60 frames per second video.
FIG. 15 illustrates how to perform such processing for finding interlaced video to be 3-2 pull down converted video. According to this processing, a field difference between a given input signal and a signal that has been delayed by two fields from the input signal is calculated. Supposing that the field difference is zero if the pictures agree with each other and that the field difference is one if the pictures disagree with each other, the difference between the same fields is calculated zero every five field. In this manner, redundant fields can be detected and the given video signal can be determined to be a 3-2 pull down converted video signal.
FIG. 16 illustrates how to carry out processing for scan-converting the 3-2 pull down converted video to progressive video. Portion (A1) of FIG. 16 illustrates the original 24 frames per second film video, while portion (A2) of FIG. 16 illustrates the 3-2 pull down converted video. The 3-2 pull down converted video (A2) is encoded by a broadcaster, carried as a broadcasting wave, and then decoded by a receiver. As a result, the signal (B), which is interlaced 60 fields per second video, is obtained. And portion (C) illustrates progressive 60 frames per second video that has been subjected to the scan conversion.
According to the conventional method, one frame is formed by combining together an odd-numbered field b1 and an even-numbered field b2 of the interlaced 60 fields per second moving picture (B), thereby generating frames c1 and c2 of interlaced 60 fields per second moving picture (C) as shown in portions (B) and (C) of FIG. 16. In the same way, respective frames c3 and c4 of the interlaced 60 fields per second moving picture (C) are formed by an odd-numbered field b3 and an even-numbered field b4 among the fields that make up the interlaced 60 fields per second moving picture (B). And the frame c5 is generated by an odd-numbered field b5 and an even-numbered field b4. By repeatedly performing this processing, the progressive 60 frames per second moving picture (C) can be obtained. The progressive moving picture (C) will have the same pictures, which have been generated from each frame of the original 24 frames per second moving picture (A1), alternately repeated in two frames and then in three frames.
Suppose the interlaced video (A2) shown in FIG. 16 has been encoded to have a frame structure (in which each frame is a combination of two video fields). In that case, when the encoded video is decoded by the receiver, the interlaced video (B) shown in FIG. 16 is obtained.
Now take a look at the fields b5, b6, b7 and b8 of the output video (B) that has been obtained as a result of the decoding processing. The fields b5 and b6 that have been encoded and then decoded in combination come from two different frames F2 and F3 of the original film video. Likewise, the fields b7 and b8 also come from two different frames F3 and F4.
Such video has no correlation between the odd- and even-numbered fields, and therefore, will achieve lower coding efficiency during encoding. As a result, the image quality of the field video, generated by decoding those fields, will be inferior to that of the field video in which video yet to be subjected to the 3-2 pull down conversion is encoded within the same frame.
The conventional technique pays no attention to those fields with deteriorated image quality when scan conversion is performed on the 60 fields per second video that has been subjected to the 3-2 down conversion.
The 3-2 pull down conversion is carried out using every four frames of the film video as one processing unit, thus generating 10 frames (e.g., c1 through c10 shown in FIG. 16) of 60 Hz progressive video per processing unit. In that case, video quality will deteriorate in five out of those ten frames (e.g., in c5 through c9 in FIG. 16), thus leaving plenty of room for improvement.